Forest Research: Open Access
Open Access

Influence of Altered Fire Return Intervals on Plant Diversity in the East Texas Pineywoods

William J. Steinley, Brian P. Oswald^*, Kathryn R. Kidd and Jessica Glasscock ^*Correspondence: Brian P. Oswald, Department of Forestry and Agriculture, Stephen F. Austin State University, Nacogdoches, Texas, USA, Email:

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Introduction

For millennia, ecosystems in North America have been impacted by fire, influencing the evolution of many species, human use of fire has also evolved over time [1,2]. Fire regimes result from the cumulative interaction of fire, vegetation, climate, humans, topography and in combination with other disturbances, are both spatially and temporally intertwined [3,4]. As a disturbance, fire is one of the most influential natural forces exerted on plant communities. In disturbance regimes, and specifically in wildland fire regimes, the presence of keystone plant species determines species composition [5,6].

Fire exclusion policies of the early 1900s in the United States altered pre-existing forest composition, resulting in excessive understory fuel loading, accumulation of hazardous ladder fuels and reduced quality of wildlife habitat [7]. Fuel conditions such as type, loading, structure, and continuity, as well as climatic variables, influence the intensity of fire events, which will also influence fuel variables following fire events [8]. The level of mortality and the frequency of fire may change due to anthropogenic fire exclusion or natural shifts in climatic factors; if the change in mortality is significant or continuous, the plant community composition will shift toward species that favor the new conditions [9,10]. The consequence of increased plant community richness and diversity following fire is pervasive, as the localized accumulation of biological heterogeneity reinforces prolonged ecosystem resistance amid dynamic disturbance regimes [11].

The pine-bluestem (Pinus-Andropogon) plant association historically dominated most of the southeastern United States, and formed a complex network of patches across the landscape which sustained frequent, low-intensity surface fires [12]. In the Southeastern United States, longleaf pine (Pinus palustris) savannahs were considered the most diverse herbaceous ecosystems in North America, due to the understory of grasses and forbs resulting from a combination of frequent high density lightning strikes and Native Americans. At the regional level, fire regime effects are complex results of longterm climatic shifts interwoven with human activities and natural disturbance regimes [13-15].

The significant reduction in longleaf pine savannahs has caused pronounced changes in plant community compositions across the south, with reduced understory diversity through increased light-intercepting midstories. Competition with midstory and overstory species reduces the effectiveness of longleaf pine natural regeneration, and without fire, nutrient recycling is slower and decreases in native graminoid diversity result from the buildup of woody debris and litter on the forest floor [16-18].

The majority of the Pineywoods ecoregion of Texas was comprised of longleaf pine and mixed longleaf-loblolly (P. taeda)-shortleaf (P. echinata)-hardwood forests. These pine species shed needles which form a fuel bed promoting fire spread and intensity; while reducing encroachment of woody species [19]. The historical fire return interval of the longleaf pine communities in east Texas has been estimated to be between 1 to 3 years, and for mixed pine-hardwood communities between 4 to 6 years [20]. Understory hardwoods and grasses found in longleaf pine and mixed-pine hardwood communities are typically dominated by species with underground rhizomes and extensive storage organs that enable them to persist after loss of aboveground biomass [21,22].

Longleaf pine-bluestem communities support a heterogeneous understory, with nearly 200 grass species identified as part of this type forbs also account for a sizable portion of the ground cover. Woody understory plants, including shrubs and vines, are frequently found in this type; however, their abundance is related to degree of disturbance [23]. Shortleaf pine has the widest distribution of any pine in the southeastern United States, and it tolerates a wide variety of soil and site conditions. Seedlings between 2-3 months of age develop a distinctive j-shaped crook near the surface, where axillary buds are formed which will initiate growth if the upper stem is damaged or killed. Due to the high fire frequency across its native range, this adaptation was likely naturally selected as a resistance to fire, and sprouting ability diminishes once diameter growth exceeds 15-20 cm in diameter [24]. Common associated species in shortleaf pine communities resemble that of longleaf pine and is linked to fire [25]. The highly adaptable loblolly pine is the dominant overstory species on nearly 11.8 million hectares from eastern Texas to the Atlantic coast in Virginia and is considered fire resistant. However, it is less fire adapted than longleaf and shortleaf pines and can be killed by moderately intense surface fires when young [26,27]. Frequent summer fires will maintain the more mature loblolly pine-grassland plant community. Crown damage generally results in greater mortality than that caused by basal damage. However, older trees tend to exhibit a greater resistance to scorch than younger trees [28]. Herbaceous cover includes bluestem, panicum (Panicum spp.), various sedges, and fennels; frequent fires in the southern pine-bluestem community increased species diversity and richness [26,29,30]. Extended fire return intervals have altered the understory composition of these ecosystems. The reduction of shade tolerant grasses is a priority on many sites in East Texas, as understory species abundance was reduced by increases in overstory cover [31].

Federal agencies such as the United States Department of the Interior, Department of Agriculture, Fish and Wildlife Service, and the Bureau of Land Management implement fire management policies which promote prescribed burning for a variety of objectives, and also adhere to the US National Fire Plan, which is a longterm fuels reduction program created with the goal of restoring the historical structure, diversity, and dynamics of forest and rangeland ecosystems [32]. Prescribed fire by private landowners, Non- Government Organizations (NGOs), and timber companies have used prescribed fire for many of the same management objectives [33].

Historically, the southeastern United States experienced recurring fires during the mid to late growing season [20]. Current application of prescribed fires is during the dormant to early growing seasons when fire behavior and intensities are more easily predicted and the risk of fire escape is more easily mitigated; this shift to dormant season burns limits control of encroaching hardwood species and over time these individuals grow large enough that the use of fire is not sufficient to cause mortality [24,33].

The goal of this research was to assess the effect which fire frequency has on understory plant community composition in East Texas. The specific objectives were to: Compare different burn regime frequencies (high/low) and their effects on understory plant community composition; analyze cover type stand characteristics and how their properties affect understory species richness and evaluate changes in species richness based on the differences in stand characteristics.

Materials and Methods

Site description

The Angelina and Sabine National Forests comprise over 121,000 hectares. Longleaf pine is the dominant cover type on the southern portion of the Angelina, while shortleaf and loblolly pine dominate the rest. The Sabine currently has a mixed cover type of shortleaf, loblolly and to a lesser extent, longleaf pine (Figure 1). Soils on the Angelina/Sabine range from sandy well drained uplands to poorly drained clayey bottomlands. Prescribed fires are implemented to aid the recovery of the endangered red-cockaded woodpecker and fuels reduction. The 5,395-hectare upland island wilderness within the Angelina National Forest was set aside in 1984, and is comprised of cover types from pitcher plant (Nepenthes gracilis) bogs to longleaf pine forests. The soils consist of well drained sandy uplands to poorly drained clayey bottomlands. The Davy Crockett National Forest covers more than 64,750 hectares, with a predominant overstory a loblolly-shortleaf mix. Hardwood species make up a substantial component of the forest, particularly near the Neches River. Soils range from deep, well drained, rapidly permeable soils on uplands to poorly drained anaerobic clays [34].

Figure 1: East Texas study areas include Angelina, Sabine, Davy Crockett National Forests, Upland Island Wilderness, The Nature Conservancy’s Sandyland Sanctuary Winston 8 Ranch.

The Nature Conservancy’s Roy E. Larson Sandyland Sanctuary (Sandyland) protects 2,288 hectares of longleaf pine savannah, to achieve this goal, prescribed fire is applied at intervals as narrow as two years [35]. An extremely biodiverse area, the preserve is connected to United States National Park Service’s Big Thicket National Preserve.

The privately owned Winston 8 Ranch is composed of 1,374 hectares of longleaf and loblolly pine forests, pastures and wetlands (Sand County Foundation, 2020). Hardwood species on site included planted wildlife mast species and a variety of native species. These areas provide habitat for white-tailed deer (Odocoileus virginianus), bobwhite quail (Colinus virginianus), and eastern wild turkey (Meleagris gallopavo). Soils are generally very deep sandy and well drained with moderate to slow permeability.

Field data collection

To determine understory vegetative species composition prior to and following prescribed burning with different fire frequencies, cover types and sites, 57 circular plots were measured across six sites in East Texas (Table 1). Plot locations were randomly selected using the ArcGIS random point tool. Plots were 0.083 ha, with a radius of 16.3 m., with three transects radiating from plot center in 120° intervals (Figure 2). Along each transect, woody species composition, herbaceous cover, downed woody debris, and litter depth was measured. Tree (≥ 5.08 cm diameter) heights were recorded and spatially referenced based on distance from plot center. Understory composition was tallied by genus or species within three, 0.3 × 0.6 m. rectangular subplots located to the right of each transect. Each subplot was marked to aid in post-fire/ recapture data collection. Using a modified version of the Brown method downed woody debris was inventoried along each transect [36].

Figure 2: Circular plot design, adapted from Wall et al.

Table 1: Total number of plots in each site, cover type (Plots with >80% species basal area), fire frequency (high>5, low<5) burns in 20 years.

Experimental design sampling

Plots were classified into low (<5 burns in past 20 years) and high (≥ 5 burns in past 20 years) fire frequency groups. Cover types (longleaf pine, shortleaf pine, loblolly pine, mixed pinehardwood) were assigned based on basal area calculations with species comprising >80% of plot basal areas representing the cover type. Normality testing using a Shapiro-Wilk test revealed a nonnormal data distribution for five variables (species richness, litter depth, fuel loading, overstory canopy cover, and hardwood basal area). These were transformed using logarithmic and square root functions. Welch t-test was conducted to compare the differences between high and low fire frequency groups. Analysis of Variance and Tukey’s post-hoc analysis was used to examine the differences among the four cover types and six sites (Davy Crockett, Angelina, and Sabine National Forests, Sandyland Sanctuary, Winston 8 Ranch and Upland Island Wilderness). Non-metric multidimensional scaling was used to display the relationships between species presence/absence and the three groups (fire frequency, cover type, site). Correlations were calculated using Pearsons’s method. All tests were conducted with a confidence level of 0.90 and performed using R version 4.3.0 [37].

Results

Fuel loading

Excluding Winston 8 Ranch, fuel loads were quantified at 50 plots (Table 2). There was no significant difference in the fuel loading of the high/low fire frequency groups (Table 3), but loblolly pine had significantly greater fuel loads than longleaf pine and the mixed pine-hardwood cover types (Tables 4 and 5).

Table 2: Summary statistics (n, min, max, mean, standard error, standard deviation, mean confidence interval) for fuel loading, litter depth, basal area, canopy cover and species richness.

Table 3: Two sample t-test analysis of high and low fire frequency groups.

Litter depth

Litter depth significantly differed between the high/low frequency and cover type groups (Tables 2 and 4), with shortleaf pine, loblolly pine, and mixed pine hardwood cover types having greater litter depths than longleaf pine. Upland Island Wilderness, Angelina, and Davy Crockett National Forests were significantly greater than the Sandyland Sanctuary (Table 5). Litter depth was negatively correlated to grass, forb, total richness and with the number of burns (Table 6).

Table 4: Analysis of Variance output measuring the difference in variable values between 4 cover type groups and 6 sites.

Table 5: Variable means measured in this study and analyzed using ANOVA.

Table 6: Pearson’s correlations between variables assessed in this study.

Overstory basal area

Hardwood basal areas ranged from 0 to 21.14 m²/ha, with an average of 6.18 m²/ha (Table 2). There was a significant difference between the hardwood basal area of high/low frequency and cover type groups (Tables 3 and 4). Mixed pine-hardwood cover types had significantly greater hardwood basal areas than the longleaf pine and loblolly pine, and Upland Island Wilderness was significantly greater than Sandyland Sanctuary (Table 5). Total, grass and forb richness and the number of burns during a 20-year period had a negative correlation with hardwood basal area (Table 6). High fire frequency sites had significantly lower softwood basal areas than low fire frequency sites), and shortleaf pine cover types lower softwood basal area than mixed-pine hardwood, but greater than longleaf pine. There was also a significant difference in softwood basal area, as Upland Island Wilderness had greater softwood basal areas than all other sites and Sandyland Sanctuary was significantly lower than all of the others (Tables 2-5). Total, grass and forb richness and the number of burns during a 20-year period again had a negative correlation with hardwood basal area (Table 6). Total basal was significantly different between the high/low frequency groups. Negative correlations with total basal area were found for total, grass and forb richness, as well as with number of burns during a 20-year period.

Canopy cover

Shortleaf pine cover type had a lower canopy cover than mixedpine hardwood, but greater than longleaf pine and was there also a significant difference between the high/low frequency groups, with high fire frequency sites lower than low fire frequency sites and there were significant differences in mean canopy cover by site (Tables 2-5). Total and forb richness had a negative correlation with canopy cover, as did the number of burns during a 20-year period (Table 6).

Understory richness

There was a significant difference between the grass richness of high/low frequency groups as the high fire frequency sites had a greater number of grasses and forbs (Tables 2-4). Shortleaf pine and pine-hardwood cover types contained significantly fewer forbs than longleaf pine and loblolly pine. And a significant difference in forb richness greater at Sandyland Sanctuary than Upland Island (Table 5). Forb richness was found to be positively correlated with the number of burns during a 20-year period (Table 6). There were significant differences in mean woody richness between sites, as Winston 8 Ranch had fewer woody growth forms than Sabine and Davy Crocket National Forests. Woody richness was not found to be correlated with number of burns in a 20-year period (Table 5). There was a significant difference between the total richness of high/low frequency groups, high fire frequency sites had greater richness than low fire frequency sites (Tables 2 and 3). Total richness was found to be positively correlated with the number of burns during a 20-year period (Table 6).

Species observations

129 species were observed across all plots (Table 7). The most observed species in the woody growth form category was American beautyberry (Callicarpa americana), and the most observed grass species was inland sea oats (Chasmanthium latifolium) with longleaf uniloa (Chasmanthium sessifolium) a close second. The most observed forb species was butterfly pea (Clitoria mariana). The most common species observed on high and low frequency sites was similar to total richness for all plots, but the most common forb observed in the low fire frequency group was elephants’ foot (Elephantopus tomentosus) (Table 8). The most common woody, grass and forbs in the shortleaf pine and mixed pine-hardwood cover types were American beautyberry, inland sea oats, and elephant’s foot. Loblolly pine cover types most common growth forms were blackberry (Rubus L spp.), panicgrass (Panicum spp.), and blue wild indigo (Baptisia australis); longleaf pine cover types most observed woody, forb, and grass were yaupon (Ilex vomitoria.), ragweed (Ambrosia artemisiifolia), and little bluestem (Schizachyrium scoparium) (Table 9).

Table 7: Species contributing to total richness by life form (woody, forb, grass) measured across all sites.

Table 8: Species contributing to total richness by life form (W=woody, F=forb, G=grass, FE=fern, Su=succulent) measured across high and low fire frequencies.

Table 9: Species contributing to total richness by life form (W=woody, F=forb, G=grass, FE=fern, Su=succulent) measured across shortleaf pine, loblolly pine, longleaf pine, mixed pine-hardwood cover type.

The most common species by growth form on the Angelina National Forest were sweetgum (Liquidambar styraciflua), little bluestem, and wild bean (Strophostyles sp.). Davy Crockett National Forest had the same common species as shortleaf pine and mixed pine-hardwood groups.

In the Sabine National Forest, the most common woody was blackberry, the most common grass was inland sea oats, and the most common forb was St. Andrews cross (Hypericum hypericoides). Sandylands’ most common woody was longleaf pine with little bluestem and ragweed. Winston 8 had the most common growth form as panicgrass, followed by woody blackberry and blue wild indigo (Table 10).

Table 10: Species contributing to total richness by life form (W=woody, F=forb, G=grass, FE=fern, Su=succulent) measured across site type (Angelina National Forest, Davy Crockett National Forest, Sabine National Forest, Sandyland Sanctuary, land Island Wilderness, Winston 8 Ranch).

Ordination

Non-Metric Multi-Dimensional Scaling (NMDS) was conducted to visualize the distribution of understory species across the fire frequency groups. To confirm that the modeling did an adequate job of representing community structure, a subsample NMDS was conducted with 15 sites which resulted in a more acceptable average stress of 0.144. Ordination displayed distinct groupings of fire frequency sites with low fire frequency sites aligned with the presence of the Chasmanthium (Figure 3). Longleaf pine cover types were strongly correlated with multiple woody, forb, and grass genera while mixed pine hardwood cover types were associated with the presence of Chasmanthium (Figure 4). Ordination also showed associations between the Davy Crockett National Forest and Upland Island Wilderness with the Chasmanthium genus. Sandyland Sanctuary and the Pinus genus showed associations which are likely due to the large amount of understory pine regeneration. The Angelina National Forest showed the most amount of dissimilarity between sites, while Sandyland Sanctuary was distinctly grouped separately (Figure 5).

Figure 3: Non-metric multidimensional scaling constructed using fire frequency as groups overlaid with significant species scores (p ≤ 0.001).

Figure 4: Non-metric multidimensional scaling constructed using cover type as groups overlaid with significant species scores (p ≤ 0.001).

Figure 5: Non-metric multidimensional scaling constructed using sites as groups overlaid with significant species scores (p ≤ 0.001).

Discussion

High fire frequency sites had lower litter depths, basal areas and canopy cover, supporting previous research on the effects of prescribed fire frequencies on stand characteristics of pine stands [38]. The most common understory species were similar on both low and high fire frequency sites. However, high fire frequency groups contained nearly twice as many grass genera and over three times as many forb genera. Others have found similar effects of increased fire frequency on pine associated understory plant compositions [11,30,39,40].

Woody plant richness was not found to be different between the high and low frequency groups, most likely due to the repeated spouting of woody plants following dormant season fires [19,22]. Due to repeated top kill but not total removal of woody genera, quantifying species richness alone may not be effective to determine the effect of fire on woody plant understory communities [41].

Stand characteristics of cover types in east Texas vary on both spatial and temporal scales [42]. Loblolly pine had greater fuel loads and the second highest total basal area of the cover types. This relationship between basal area and fuel load has been previously reported [43]. Litter depths were greatest on loblolly, shortleaf and mixed pine hardwood types. The high continuity and flammability of longleaf pine litter provides a more homogeneous substrate for fires which reduces unburnt patches of understory and spatially less litter [44].

Total overstory basal areas were greatest on mixed pine hardwood cover types, followed by loblolly and shortleaf pines. The cause for this is multi-factorial; longleaf pine cover types were located on more well drained sites and included bluejack (Quercus incana Bartram) and blackjack (Q. marilandica Muenchh.) oaks and various hickories. Hardwoods on these sites are commonly smaller in diameter than hardwoods (sweetgum, southern red oak (Q. falcata michx.), etc.) found on more productive sites. The repeated effect of increased fire intensities appears to suppress shrubs and saplings, leading to lower recruitment and associated increases in basal area [21,43].

Hardwood basal areas were greater on mixed pine hardwood types than longleaf pine or loblolly pine, while similar to shortleaf pine. This similarity between shortleaf pine and mixed pine-hardwood stands may be due to the progression over time of shortleaf cover types into larger diameter stands with a hardwood component [16]. Softwood basal areas were greatest on the mixed pine hardwood cover type compared to all others, while longleaf pine types were lower than both loblolly and shortleaf pine. The lack of fire for decades on the Upland Island Wilderness have prevented fires from causing stem mortality and have contributed to the high basal areas [17]. Overall canopy cover was greatest on mixed pine hardwood sites, with longleaf pine cover types containing the lowest canopy cover. Fire and resulting gap formations play a role in the regeneration of longleaf pine, and the increased flammability of longleaf pine likely contributes to gap formation and the creation of savannah like plant communities [4,21].

There was no difference in grass richness between cover types, but forb richness was greatest on longleaf and loblolly types, possible driven by managers using growing season burns more frequently in these types [44]. Total and woody richness was not found to be different between cover types, and burning at intervals greater than 2 years may promote woody species establishment in pine understory communities [45].

Hardwood basal areas were not found to be different between the three National Forest sites, while Upland Island Wilderness had a greater hardwood basal area than Sandyland Sanctuary. Softwood and total basal areas were much greater on Upland Island Wilderness and much lower on Sandyland Sanctuary compared to the National Forest sites. Intensive management at Sandyland with the goal of lowering basal areas have previously been shown to benefit longleaf pine regeneration in uneven aged stands [46]. Upland Island Wilderness had been without intensive management for many years, so the introduction of growing season fires may help lower basal areas.

Canopy cover at Upland Island Wilderness were greater than Sandyland, Winston 8 and the Angelina and Davy Crockett National Forests, but not different than on the Sabine. These results are due to the different management strategies used to achieve management goals. The lower canopy cover allows increased light penetration and greater recruitment of longleaf pine seedlings, while the higher canopy cover observed at Upland Island Wilderness is a consequence of fire exclusion and has suppressive effects on forb production [4].

Fuel load, litter depth, basal area, and canopy cover contribute to changes in understory plant communities. Greater litter depths are associated with lower richness, likely because of the suppressive effect increased litter depths have on the establishment of new grasses and forbs in pine understories [47]. Litter depths decreased linearly in response to increased burning over a 20-year period, aligning with previous studies. Previous studies have also shown that increases in basal area and canopy cover led to lower species richness through increased competition for sunlight and space [31,48].

Increases in canopy cover were found to be correlated with lower forb richness, while differences in grass richness remained insignificant, possibly due to the establishment of understory grasses which thrive in shaded environments. The relationship between the number of burns in a 20-year period and the richness of forbs and grasses was found to be linear due to the direct and indirect effects of repeated fires on litter depth, basal area and canopy cover [38,48,49].

We found that the Chasmanthium species skewed strongly towards low fire frequency sites while high fire frequency sites had greater species diversity. There was a strong relationship between longleaf pine and multiple plant genera including Schizachyrium, while Chasmanthium aligned with both mixed pine-hardwood and shortleaf pine and Rubus skewed towards loblolly pine plots. Sites were not as dissimilar as expected, the exception being Sandyland Sanctuary, likely in part due to the geographic separation and unique management structure of Sandyland Sanctuary in comparison to the other sites. The Chasmanthium genera associated with Upland Island Wilderness and the Davy Crockett National Forest, while Sandyland Sanctuary had greater association with the Ambrosia, Carya, Quercus and Pinus genera: Winston 8, Angelina and Sabine National Forests were all associated with Rubus, Baptisia, and Rhus genera.

Conclusion

Overall, the plant diversity in East Texas increased through the implementation of high frequency prescribed fire, shifting stand characteristics regardless of cover type or site to favor the development of diverse understories. The fire dependent nature of these communities demands frequent fires to reduce the vigor and establishment of shade tolerant hardwood species and their stress on overstory pines as well as interception of light to the understory. The intentional reduction of fire frequency in these communities has been the greatest disturbance over the past 100 years. The re-establishment of historical fire frequencies alone may not be enough to shift these communities back to desired or historical compositions. However, the how land managers accomplish this and at what point satisfactory restoration is reached remains to be determined.

Acknowledgement

We thank the individuals associated with the various agencies and private organizations who assisted with the development and execution of this project: Matt Laricos, Ike McWhorter, Joey Silva, and Kyle Sommers of the United States Forest Service; Charlotte Reemts and Shawn Benedict of the Nature Conservancy; and Paul Wood of the Winston 8 Ranch. Our sincere appreciation is extended to Wyatt Bagwell, Cassady Pennington, Wesley Danheim, Madison Litzenberger, and Lexi Cook and all of the students over the years that collected data used in this study. Funding was provided by the McIntire-Stennis Cooperative Research Program and the Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University.

Author Contribution

All authors contributed towards project conceptualization and design, and have approved the final manuscript. WS wrote the initial draft and performed the majority of analysis. WS and BO were the primary authors of the manuscript, and KRK and JG provided substantial edits to the manuscript.

Funding

Funding was provided by the McIntire-Stennis Cooperative Research Program and Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University.

Conflict of Interest

The authors declare they have no conflict of interest.

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